1. Field of the Invention
The invention relates to the field of
2. Description of the Prior Art
There are approximately 20-40 million people in the United States with sleep apnea. The majority of them are undiagnosed and untreated at this time. Sleep apnea can lead to severe health complications including hypertension, heart failure, memory impairment, motor vehicle and work accidents, decreased work productivity, and increased risk of death. The diagnosis and management of sleep apnea currently requires polysomnography, which is complex, time-consuming; expensive, and of limited availability. The development of a novel, simple, rapid, minimally invasive method for the diagnosis and optimization of treatment of patients with obstructive sleep apnea would be a tremendous advance for these millions of patients.
In the last twenty-five years, obstructive sleep apnea has been recognized as a very common disorder and an important cause of morbidity and mortality. Obstructive sleep apnea is characterized by repetitive interruptions of breathing during sleep due to the collapse of the upper airway. The cessation of airflow usually lasts 10 seconds or more, which is defined as apnea. While 24% of males and 9% of females have 5 or more apneas per hour, the prevalence of more severe forms (more than 15 apneas per hour) has been shown to be 12% in men and 5% in women. Airway obstruction during sleep may occur at one or more sites in areas of the nasopharynx, oropharynx and hypopharynx. For the majority of patients with obstructive sleep apnea, airway closure occurs most commonly in the oropharynx region.
The abnormalities may take the form of steady snoring, protracted hypopneas with intermittent arousals or self-perpetuating transient obstructive events that recur over minutes, hours, or the entire sleep time. Frequently, these various manifestations occur in the same individual at different times. Due to repetitive cycles of snoring, airway collapse, and arousal, patients with obstructive sleep apnea suffer from fragmented sleep, chronic fatigue, daytime sleepiness, lack of concentration, and memory problems. The wide range of consequences of obstructive sleep apnea include hypertension, impotence, increased risk of motor vehicle accidents and the development of cardiovascular diseases such as right and left ventricular failure, myocardial infarction, and stroke.
The current diagnostic gold standard is in-laboratory, full overnight polysomnography which is performed to confirm the presence of upper airway closure during sleep and to assess the patient's level of risk. The polysomnogram study consists of recordings of arterial oximetry, respiratory effort, naso-oral airflow, snoring, electrocardiography and of neurophysiological variables including electroencephalogram (EEG), bilateral electro-oculogram (EOG), submental electromyogram (EMG), and bilateral anterior tibialis EMG for diagnosis of obstructive sleep apnea. Unfortunately full sleep studies are expensive, inconvenient and unable to localize and map the upper airway obstruction sites in obstructive sleep apnea patients which is important in choosing the appropriate treatment, especially for surgical intervention.
Nasal continuous Positive Airway Pressure (CPAP) treatment is the most effective and widely used method for treating obstructive sleep apnea. Through the use of a snugly fitting nasal mask, CPAP provides a gentle flow of positive pressure air to keep the airway open during sleep. The optimal titrated pressure which is the air pressure just high enough to prevent most apneas and hypopneas is determined after review of overnight comprehensive polysomnography study with progressively increasing airway pressures supervised by a sleep technician in a sleep laboratory. A lack of response to the conservative treatment qualifies a patient for surgical correction of the offending anatomical site in obstructive sleep apnea. The obstructed tissue is removed or shrunken to increase the size of the upper airway thereby preventing collapse of the airway and making breathing easier:
Our understanding of the human in-vivo upper airways activity during normal breathing and especially in sleep disordered breathing is limited. Upper airway imaging techniques routinely used include endoscopy, nuclear magnetic resonance imaging (MRI), computed tomography (CT), X-ray cephalometry, acoustic reflection, and fluoroscopy. However, X-ray cephalometry, CT and fluoroscopy all involve exposure to potentially hazardous radiation. MRI is cumbersome, expensive, noisy, claustrophobic and even impossible for patients who have contraindications to MRI. As a result, X-ray cephalometry, CT, MRI and fluoroscopy are impractical for continuous overnight studies.
Endoscopy is not associated with radiation but it requires subjective visual outlining of the airway wall for evaluation of the upper airway dimensions. Acoustic reflection is noninvasive, however it can be only performed in the sitting, instead of the supine, position, and is incapable of high resolution anatomical imaging. Due to these limitations, current upper airway imaging methods are unable to confirm or exclude obstructive sleep apnea with adequate sensitivity and specificity and therefore are not part of the routine diagnostic evaluation for obstructive sleep apnea.
Research advances with OCT have been widely used in opthalmology and dermatology. The first in-vivo endoscopic OCT images in animals and humans were reported in 1997. Thereafter endoscopic OCT has been rapidly developed for intravascular accessing and imaging of respiratory, urogenital, and GI tracts. OCT takes advantage of the short coherence length of broadband light sources to perform high resolution (about 10 μm), high sensitivity (about 100 dB), cross-sectional imaging of biological tissues. It is analogous to ultrasound B-mode imaging, but uses laser light reflectance, rather than sound as its basis. In OCT, light is emitted from a low coherence source and coupled to an interferometer where the light is split into two paths. The laser light from the low coherence source is emitted over a broad range of wavelengths that is defined by the coherence length. After being split, one beam is directed toward the sample material and the other to a reference mirror. Light backscattered by the sample is recombined with reflected light from the reference mirror to produce an interference pattern only for coherent photons that have an optical path length difference between reference and target that matches to within the source coherence length (10 μm). Hence, the recorded interference signal at the photodetector corresponds to a specific depth within the test material and results in high axial spatial resolution.
To perform depth scans in time domain OCT systems, the reference arm length is progressively increased by moving a reference mirror. Moving the scanning mechanism laterally (or rotationally) across the sample produces two-dimensional and three dimensional images. The coherence length of the light source determines the axial resolution of the system, while the lateral resolution is determined by the optical design of sampling probe or catheter.
Optical coherence tomography (OCT) is an imaging modality to perform cross section view. OCT is analogous to ultrasound except that imaging is performed with light instead of acoustic waves. OCT is non invasive and non ionizing allowing study over lengthy periods during both sleep and wakefulness. Conventional OCT which is based on time domain technique has very limited imaging speed which precludes its use in real-time, dynamic monitoring and large volume detection.
OCT systems have also been described, that through manipulation of the rapid scanning optical delay (RSOD) line configurations, can provide longer range OCT images with larger scale quantitative information about the lumen size, and shape of the upper airway. These systems can produce anatomical upper airway images with minimal invasiveness allowing study over lengthy periods during both sleep and wakefulness, and have shown the potential for studies of airway collapse during sleep apnea. However, the reported studies use time domain (TD) techniques with limited speed and sensitivity and can only achieve an imaging speed of less than 3 frames per second—which precludes its use in real-time, dynamic monitoring and large volume detection such as three dimensional imaging over the entire upper airway. In addition, motion artifacts of the airway during respiration would result in image blurring in low speed systems.
The demonstrated TDOCT systems can produce anatomical upper airway images with minimal invasiveness allowing study over lengthy periods during both sleep and wakefulness, and have shown the potential for studies of airway collapse during sleep apnea. However, the reported studies use time domain techniques with limited speed and sensitivity and can only achieve an imaging speed of less than 3 frames per second—which precludes its use in real-time, dynamic monitoring and large volume detection such as three dimensional imaging over the entire upper airway. In addition, motion artifacts of the airway during respiration would result in image blurring in low speed systems
For example, J. J. Armstrong et. al., “Quantitative upper airway imaging with anatomic optical coherence tomography”, Amer. J. Respir. Crit. Care Med., 2006. 173: p. 226-323, discloses a time domain optical coherence tomography (TDOCT) system that can provide long range OCT images with large scale quantitative information about the lumen size, and shape of the upper airway through manipulation of the rapid scanning optical delay (RSOD) line configurations.